It is good that you are careful to prevent rust on your Impala. I see a lot of rusty GM vehicles out and about.
Why specifically GM? I’ve seen plenty older GM vehicles that have little to no rust and I’ve seen plenty of non-GM vehicles with lots of rust. My dad’s ’00 Impala with over 315k miles has almost no body rust. My ‘01 Century with over 260k miles has as much rust as you’d expect from a 16 year old car that drives over 80 miles each day. It has little to do with the manufacturer. It just depends on how well you take care of the paint.